(A) Field of the Invention
This invention relates to metal coating and more particularly to solid metal coating.
(B) Prior Art
Processes for producing heavy metal crystalline phosphate coatings on ferrous metal surfaces to insure good bonding of subsequent paint coats are well known in the art. Zinc phosphate in particular has been in general use for decades for this purpose, although other metals, including cadmium, calcium and manganese phosphate have been suggested.
Several problems are associated with phosphate coatings. The phosphate coating of metal so as to achieve the desired end metal product is not a simple process. One difficulty has been found to lie in the peculiar properties of heavy metal phosphates themselves. For example, as would ordinarily be expected, deposition coating of a metal surface is accomplished with more facility the higher the temperature due to an increasing chemical activity with increasing temperatures. Heavy metal phosphates, however, have the property of inverse solubility. That is, their solubility decreases as temperature increases. Since low solubility also means greater ease in securing a deposition type coating, the coating process is thus doubly expedited when performed at elevated temperatures. The boiling point of the acid phosphate solution has been a common temperature. Unfortunately, heated solutions are more difficult to handle, require larger expenditures of power to keep them in the heated state, and tend to precipitate out on heating coils or the hotter parts of the retaining tanks.
A second problem, especially with zinc phosphate coatings relate to their high price.
These problems being well known in the art for years, there has been an ongoing, long felt desire to perfect a coating process which may be operated over a wide range of temperatures, especially at temperatures lower than the boiling point of the phosphate solution and which will, secondly be economical.
Unfortunately in regard to using a lower temperature, the result is a greater solubility of metal phosphate, with concurrent reduction of metal phosphate available to deposit.
One 1940's prior art reference, U.S. Pat. No. 2,316,811 proposes solving the high solubility at low temperature problem by raising the pH such that a supersaturated solution of at best 20% is produced at the temperature (60.degree. F. to 129.degree. F.) for the concentration of metal phosphate utilized, thereby maximizing the amount of metal phosphate available to be deposited. Such a procedure raises a host of new problems inasmuch as the pH of the coating solution is itself critical for good results. First of all, any supersaturated solution is by its nature only semi-stable and consequently highly subject to desaturating with resultant sludge production. Furthermore, at this high a pH no coating will result even though the solution is supersaturated with metal phosphate because at very high pH's there is no free acid (H.sup.+) present anymore. The lack of free acid (H.sup.+) being present is fatal to a satisfactory rate in the deposition process because the acid initiates the process of depositing the phosphate coating on the metal surface. The aforementioned reference overcomes this rate problem by utilizing an oxidizing agent as an initiator to generate the requisite initial free acid. This same reference is directed toward zinc phosphate solutions, but discloses calcium, cadmium, and manganese as heavy metal equivalents presumed utilizable in the same manner, although no examples of calcium are disclosed.
A few years later, German Pat. No. 741,937 indicates the reason for the total lack of examples in regard to calcium phosphate. Although this latter reference attempted calcium phosphate at lower than boiling temperatures and with what appears to be both supersaturated and non-supersaturated solution, ultimately the only satisfactory adherent coating utilizing calcium phosphate was achieved at the previously known prior art combination of high temperature (98.degree. C) and low (2.62) pH. On the other hand, low temperatures and high pH's were disclosed as producing satisfactory zinc phosphate coating, thus confirming the earlier work in regard to zinc phosphate. Low temperature, adherent calcium phosphate undercoats remained an unsolved problem.
Since this very early work, much additional work has been done to overcome various problems with, for example, the oxidizers (for example U.S. Pat. No. 2,351,605). Other than this, some coatings are known today which combine zinc and calcium phosphate. However, no methods are known which disclose the production of calcium phosphate coatings especially processes utilizable over a wide temperature range. Zinc phosphates, in spite of their higher price, have thus reigned supreme and unchallenged.